Hydrogen has been spotlighted as an ultimate clean energy source that, when burned, unlike a fossil fuel, does not produce a substance, such as carbon oxide or a sulfur oxide, which may cause an environmental problem. A heat capacity per unit mass of hydrogen is at least three times as much as that of petroleum. Accordingly, if hydrogen is provided to a fuel cell, hydrogen can be converted at a high efficiency to electronic energy or heat energy.
A chemical hydrogen-production method conventionally proposed is a technique such as thermal cracking and steam reforming for natural gas or naphtha. In this method, reaction conditions of a high temperature a high pressure are required. Synthetic gas produced according to this method contains carbon monoxide (CO). Accordingly, in a case where the synthetic gas is used as a fuel for a fuel cell, it is necessary to remove CO for preventing poisoning caused by CO in an electrode catalyst of the fuel cell. However, removal of CO from the synthetic gas is a technically very difficult problem.
As another hydrogen production method, there is a biological hydrogen-production method using a microorganism. The biological hydrogen production method using a microorganism is advantageous in, for example, that reaction conditions of the method are a normal temperature and a normal pressure and that, because produced gas does not contain CO, removal of CO is not necessary. In view of this, the biological hydrogen-production method using a microorganism is spotlighted as a more preferable method for supplying a fuel to a fuel cell.
The biological hydrogen-production method is broadly classified into a method using a photosynthetic microorganism and a method using a non-photosynthetic microorganism (mainly, an anaerobic microorganism). The former method using a photosynthetic microorganism utilizes light energy for hydrogen production. This former method has a low utilization efficiency of light energy and requires a larger area for collecting light. As a result, a hydrogen production device according to the former method has problems to be solved, for example, high cost for a hydrogen production device and difficulty in maintenance. These problems prevent practical implementation of the method using a photosynthetic microorganism.
The latter conventional hydrogen production method using an anaerobic microorganism is a method relying on multiplication of an anaerobic microorganism. In this method, there has been a problem such that a large reaction container is required because a speed of multiplication of the anaerobic microorganism is extremely slow under an anaerobic condition (U.S. Pat. No. 5,834,264 (issued on Nov. 10, 1998)). In view of this problem, a group including an inventor of the present invention has developed a hydrogen production method in which a speed of hydrogen production per unit volume is improved (International Application Publication No. WO2004/074495 A1, pamphlet (published on Sep. 2, 2004)). According to the method described in International Application Publication No. WO2004/074495 A1, pamphlet, when formic acid is used as a material for producing hydrogen, the formic acid added is not retained within a reaction section but immediately decomposed into hydrogen and carbon dioxide.
Another group including the inventor of the present invention discloses that, by culturing a microorganism under control of a concentration of organic acid and/or alcohol in a culture solution under an anaerobic condition, it is possible to cause induced expression of an ability of producing hydrogen in a microorganism that originally does not have the ability of producing hydrogen (Japanese Patent Application Publication, Tokukai, No. 2006-55127 (Publication Date: Mar. 2, 2006)). According to the method described in Japanese Patent Application Publication, Tokukai, No. 2006-55127, a microorganism having an ability of producing hydrogen can be efficiently prepared. In addition, Japanese Patent Application Publication, Tokukai, No. 2006-55127 discloses that: (i) when formic acid is used as a substrate for hydrogen production, a high formic acid concentration is preferable in view of easy control of an amount of liquid of a reaction solution; and (ii) the formic acid concentration is preferably not less than 30% (w/w) and less than 100% (w/w) in an organic material to be supplied.
The another group including the inventor of the present invention has also found that, under control of a formic acid concentration to 250 mmol/L or less in a reaction solution, continuous hydrogen production can be achieved while a cumulative amount of produced hydrogen is not decreased (Japanese Patent Application Publication, Tokukai, No. 2006-333767 (Publication Date: Dec. 14, 2006)). Further, the another group has further found that, for the purpose of preventing a decrease in the cumulative amount of produced hydrogen and continuously producing hydrogen, the formic acid concentration in the organic material to be supplied is controlled more preferably within a range of 0.5 mol/L to 24.0 mol/L while a culture solution is circulated (Japanese Patent Application Publication, Tokukai, No. 2007-330113 (Publication Date: Dec. 27, 2007)).